Recently, manufacturers of virtually every type of electrical and electronic component have made significant strides in reducing component size. As sizes were reduced, it became increasingly important to assure that the fabrication of the components was free from defect. Defects on these increasingly small components could range down to a size of a few microns, or less. Cracks and scratches in glass substrates can have widths of less then 0.0003 mm, yet still propagate during the manufacturing process. Defects of even this size will therefore render many of today's electronic components inoperable.
Examples of electronic components requiring defect free surfaces include flat panel displays (FPD's) such as active matrix liquid crystal displays (AMLCD's), optical disks, optical flats, etalons and precision windows, prisms, and laser mirror substrates, to name but a few. Specific materials, such as transparent coated optics for use in LCD's and solar cells, also require surfaces that are free from defect. Added to the fact that defects in these materials can be as small as micron, is the fact that these devices or surfaces can have areas of one meter or more.
With respect to FPD's and particularly to AMLCD's, inspection is critical in that manufacturers report manufacturing yields of such devices at less then forty percent. One area which would provide substantial improvement to manufacturing yields would be to improve the quality of inspection of glass substrates prior to beginning the manufacturing process, after the deposition of preliminary layers of material, and during the overall process to screen for dust. Initial inspection of the substrates would screen for breaks, chips, cracks, warpage, and for veins and bubbles. Other key inspection criteria are for dust, scratches, and organic dirt.
It has long been known that the best way to inspect large area, high quality surfaces is to use light directed at the surface, and measure the amount of scattering, if any, caused by specific defects in the surface under investigation. A major problem has resulted from instances in which light beams reflected off of minute defects are scattered so slightly as to be indistinguishable from "noise" in the monitoring system. These types of situations typically arise when the defect is smaller than the diffraction limit of the light based system. Similarly, these problems will occur when the depth of the defect is less than the wavelength of the illuminating beam. These limitations in light scattering systems have rendered them unfit for inspecting extremely high resolution devices, since minute defects (although large enough to render the device inoperable) are undetectable by the system.
In an effort to overcome these shortcomings in light scattering systems, interferometric systems have been devised. Interferometry refers to measuring differences in the time or phase of two or more signals received by sensors spaced a known distance apart. These types of interferometric systems are widely known, and in laboratory usage today. However, a major limitation of interferometric systems is that detection of the phase must be done in the specular direction since the wave fronts emerging from the source and the defect are overlapping in space (i.e., are propagating together). This results in a high DC level which must be overcome at the detection of the signal. Phase contrast interferometric methods overcome the problems associated with high DC level, but are extremely sensitive to adjustments, vibrations, mis-alignments, etc. Further, interferometric systems have fields with cyclical variations, and consequently a large oscillating phase term. Hence, the surface calculation becomes ambiguous since any point of the field can be interchanged with another, one wavelength distant. An additional limitation is the necessity for a reference beam, making the entire system sensitive to the reference adjustment within the space of a single wavelength.
These types of interferometric systems have proven successful, particularly in laboratory applications, since they possess heretofore acceptable sensitivity to defects, and are capable of discerning defects from background noise. Nonetheless, these types of systems have not proven useful in industrial settings. This is because their high sensitivity is easily effected by external influences, such as vibration and system adjustments.
Attempts have been made to eliminate these types of problems. The usual method of removing high DC levels present in interferometric systems is to apply phase contrast interferometry. This method provides adjustments to the two interfering beams of light so that the nominal optical path difference inside the system is one half of a wavelength. Consequently, after interference, the resulting amplitude is zero. Thus, a small change in one of the two interfering beams will result in a significant change in the resulting amplitude, and hence a significant measurement signal, free from a DC component.
The advantage of the phase interferometric system resides in the fact the phase contrast has a "zero" response to a "zero" signal, while a regular interferometric system has a "non-zero" reference level. The disadvantage of the phase contrast method is its extreme sensitivity and inflexibility to adjustments in the system. Any change in the nominal path length in the system will cause both a non-zero DC level, and a change in the signal contrast at the detector. This problem is exaggerated when high speed scanning is taking into account, as in industrial applications.
Further, and of major importance to manufacturers of FPD's and particularly to AMLCD manufacturers, is the fact that resolution levels for these type of systems remains below acceptable levels. For example, spatial resolution and depth resolution for conventional interferometers is approximately 2.0 .mu.m and 0.01 .mu.m respectively, while being extremely slow and unstable.
Accordingly, there exists a need for a light based inspection system with very high sensitivity to minute changes in the optical path (and hence the presence of defects on the investigated surface). The inspection system should be able to detect defects below the diffraction limit of the optical system, and be able to do so in a relatively fast manner. The inspection system should also have little or no influence from external factors.